This patent application is related to a commonly assigned U.S. patent application filed on even date herewith entitled "Oil-Free Liquid Chiller" as well as commonly assigned and allowed U.S. patent application Ser. No. 08/965,495, now U.S. Pat. No. 5,848,538 entitled "Oil and Refrigerant Pump for Centrifugal Chiller" and any divisional applications that may derive therefrom.
The present invention relates to liquid chillers. More particularly, the present invention relates to refrigeration machines of the centrifugal type the purpose of which is to cool a liquid, most typically water, for use in building comfort conditioning or industrial process applications. With still more particularity, the present invention relates to a centrifugal refrigeration chiller having significantly enhanced motor cooling and lubrication arrangements.
Refrigeration chillers are machines that employ a refrigerant fluid to temperature condition a liquid, such as water, most often for purposes of using such liquid as a cooling medium in an industrial process or to comfort condition the air in a building. Refrigeration chillers of larger capacity are typically driven by compressors of the centrifugal type resulting in the denomination of such machines as "centrifugal chillers".
Centrifugal compressors are compressors which, by the high speed rotation of one or more impellers in a volute housing, act on a refrigerant gas to compress it. The impeller or impellers of a centrifugal compressor, the shaft on which they are mounted and, in the case of so-called direct drive compressors, the rotor of the compressor drive motor, weigh hundreds if not thousands of pounds. The relatively high speed rotation of such physically large and heavy chiller components at several thousand RPM presents unique and challenging bearing lubrication issues. Likewise, the heat developed by the motor which drives such components is significant and the temperatures associated with motor operation can be relatively very high, particularly under certain operating and load conditions. As a result, proactive cooling of the compressor drive motor is required.
Centrifugal chiller lubrication and motor cooling arrangements are generally well developed. However, there is ever increasing pressure to increase the overall efficiency of such chillers which are typically among the largest energy users in a building or industrial process. At the same time, restrictions on the kinds of refrigerants that can be used in such chillers have been established due to environmental concerns.
The characteristics of newer, more environmentally friendly refrigerants are such as to have the effect of potentially reducing the effectiveness and reliability of chiller motor cooling systems. This is because such newer refrigerants are lower pressure refrigerants and the use thereof results in significantly decreased pressure differentials across the chiller systems in which they are employed, particularly when certain operating conditions exist. Such pressure differentials have historically been used to cause or assist in the movement and delivery of refrigerant to a chiller's compressor drive motor for motor cooling purposes.
For example, in current chillers manufactured by the assignee of the present invention (assignee being the largest manufacturer of such chillers in the world) which employ newer, low pressure refrigerants and which rely on chiller pressure differentials to move refrigerant, a limit is imposed on so-called low head operation to ensure that refrigerant is both delivered to and returned from the motor location whenever the chiller is operating. The low head limit is a differential pressure, as measured between the high pressure and low pressure sides of the chiller system, which is minimally sufficient to ensure the supply and return of refrigerant to a chiller's compressor drive motor when the chiller is operating. In certain present chillers, the low head limit is approximately 5 psi.
While the low head limit is typically not reached, it can be reached under certain relatively infrequently occurring operating conditions where newer, low pressure refrigerants are employed. The existence of such conditions, even if only infrequent and/or transitory, can result in periods of chiller shutdown to avoid motor overheating during which the chiller will not produce the chilled liquid which is necessary to the purpose for which the chiller is employed. Where a chiller is used to comfort condition air in a large factory or a commercial, government or school building or the like or where a chiller is used in an industrial process that relies upon a continuous supply of water which is chilled to a specified temperature for production of an end-product, such as computer chips, chemicals or the like, chiller downtime is to be avoided if at all possible.
Because current systems operate based on the existence of the pressure differential between the source location for refrigerant, the location of its use (the compressor drive motor) and/or the location to which it is returned from after such use, the location of use must be at a pressure lower than the pressure at the source location. In the case of prior and current centrifugal chillers, refrigerant used for motor cooling is typically driven through an orifice from the relatively high pressure condenser of the chiller to the housing in which the compressor drive motor is housed where the refrigerant is brought into contact with the motor in order to cool it. The orifice acts as a pressure boundary between the relatively high pressure condenser and (1) the lower pressure motor housing and (2) the location to which the refrigerant is returned from the motor housing.
Because a significant portion of the liquid refrigerant driven from the condenser to the motor will flash to gas in its passage through the orifice and prior to having any motor cooling effect, the refrigerant delivered to the motor for motor cooling purposes in such systems is much less effective for that purpose than would be the case if it were delivered to the motor entirely in the liquid state. As such, while current motor cooling arrangements are, in fact, effective, the actual cooling effect of the refrigerant driven to a drive motor and overall chiller efficiency is significantly degraded as a result of that refrigerant's gas content.
As a result of demands for increased chiller efficiency and for chiller motor cooling systems that do not make use of or rely upon pressure differentials that may or may not exist in the chiller under certain operating conditions, particularly with the advent and use of newer refrigerants, the need exists to provide for a motor cooling system that operates across the entire operating range of the chiller and which acts to minimize the chiller efficiency loss that results from the motor cooling process. In conjunction with such change to chiller motor cooling arrangements and because (1) a certain amount of refrigerant will make its way into the chiller's lubrication system and (2) a certain amount of lubricant will make its way into the chiller's refrigeration circuit, the need and opportunity also exists to improve chiller lubrication systems so as to make them more reliable, to enhance the return of oil which finds its way into the chiller's refrigeration circuit back to the chiller's lubrication system and to maintain such oil therein.